Field of the Invention
The present invention relates to an inverter control circuit for controlling an operation of transferring the power from an inverter to a reserve power source.
Description of the Prior Art
A conventional inverter control circuit for such a purpose as shown in FIG. 1 has been proposed. In FIG. 1, there are shown an inverter 1, a commercial power source 2 (reserve power source), thyristor switches 3 and 4, a load 5, a crystal oscillator 11, an inverter voltage control circuit 12, Schmitt trigger circuits 21 and 22, a phase difference sensing circuit 23, a control amplifier 31, a condenser 32, a V/F converter 51, and a pulse adder-subtracter circuit 52. The Schmitt trigger circuits 21 and 22, the phase difference sensing circuit 23, the control amplifier 31 and the condenser 32 constitute a synchronism control circuit S. In FIG. 1, a symbol .phi. indicates a voltage phase difference signal.
The manner of operation of the conventional inverter control circuit of FIG. 1 will be described hereinafter.
FIG. 2 shows a mode of a phase difference signal .phi. provided by the voltage phase difference sensing circuit 23 during the operation of the Schmitt trigger circuits 21 and 22 and the phase difference sensing circuit 23. A voltage phase difference .phi..sub.1 at a time t.sub.0 is reduced to zero at a time t.sub.1 by the operation of a digital PLL (Phase Locked Loop) including the inverter voltage control circuit 12, the V/F converter 51 and the pulse adder-subtracter circuit 52. Thereafter, the phase difference remains zero as long as the voltages do not change suddenly. When a trouble occurs in the inverter 1 in such an operating state, the thyristor switches 3 and 4 are controlled so as to transfer the load 5 uninterruptedly from the inverter 1 to the commercial power source 2.
At the same time, the frequency of the digital PLL circuit is converted into a pulse frequency corresponding to a control signal provided by the control amplifier 31 to the V/F converter 51. The converted pulse frequency is added to or subtracted from a reference oscillation pulse frequency of the crystal oscillator 11 by the pulse adder-subtracter circuit 52. The output frequency of the inverter 1 is controlled by giving the output pulse signal of the adder-subtracter circuit 52 to the inverter voltage control circuit 12 as a clock signal.
When a trouble occurs in the synchronism control circuit, the above-mentioned conventional inverter control circuit is unable to synchronize the inverter with the commercial power source. Accordingly, in repairing the synchronism control circuit, the power source should be transferred at once from the inverter to the commercial power source. But since this transfer operation is to have a short break of power because of non-synchronism operation, non-interrupting the operation of the load is impossible.